Segmented carbon radial or circumferential seals have been employed in a number of environments associated with compressible fluids, such as gases. They have been used, for example, in connection with gas turbine engines. Such radial seals typically act to, among other things, seal high pressure areas from low pressure areas. For example, such seals have been used for inhibiting oil or oil-air mist from leaking into the air system in bearing locations.
Performance of segmented seals is commonly dependent upon shaft speed and contact pressure between segmented seals and a shaft, where the most challenging condition is experienced at maximum speed and pressure. The life of conventional contacting seals is generally characterized by a parameter referred to as a “PV” value. The PV value is generally defined as the product of the seal surface speed and the surface contact pressure. With segmented seals, the surface contact pressure consists of two parts—the unbalanced pressure force in the radial direction and the mechanical spring (e.g., garter spring) closure force. For dry running seals the PV limit may, for instance be limited to, approximately 10,000 psi-ft/min. Since many of the parameters—such as system and discharge pressures and shaft speed—are commonly dictated by application for segmented seals, the principal way to remain below the PV limit of the material is to reduce overall contact pressure by reducing dam width. Consequently, many conventional seals rely solely on a geometrical means for reducing PV.
Another potential issue with conventional segmented radial seals is the amount of heat generation, which is a function of contact pressure and shaft speed. If the environmental temperature of fluid flow does not allow the heat to be transferred away effectively, then the heat generated by the seal can result in coking around the seal or excessive seal wear, which can eventually contribute to seal failure.
Attempts have been made to address the foregoing issues but have resulted in only limited success in the reduction of PV limitations. Some designs employ long and shallow circumferential grooves and high pressure vent holes to create a hydrodynamic lift force to help reduce contact pressure. This can provide good opening force near the trailing end of the segment. However, the hydrodynamic force on the leading end of the segment would be minimal. An overall lift would necessitate nearly perfect collaboration from all segments of the seal which may not be practical or likely.
Other designs attempt to use standard Rayleigh pads to generate hydrodynamic opening forces to enable the seal to operate in a non-contacting condition. With such “static system pressure feed” designs, multiple pads may need to be provided on a single segment. With this type of approach, the opening force can be more evenly distributed along the circumferential direction. However, in the application of Rayleigh pads in a circumferential sealing element, such as a segmented seal or any sealing element in which the primary portion of the seal is in intimate contact with a rotating element (e.g., a shaft), there is a reliance on the rotation of the shaft to feed the pad. There are losses that affect the pad's ability to generate and hold hydrodynamic pressure when used in the circumferential sealing application. The fluid enters the deep groove axially along the shaft adjacent to the pad, and the deep groove is the fluid supplier to the pad. The fluid that enters the deep feeding groove must turn 90 degrees from the direction of rotation to enter the Rayleigh pad area. Such a sharp turn of the fluid to enter the pad and the abrupt transition at the entrance are where losses can occur.
However, even with such recent advancements, there are commonly shaft speed limitations and, for some designs, the configuration may prevent fluid from adequately feeding the pad when above certain relative speed ranges. With surface imperfections and entrance losses, this could reduce the cavity pressure to value below the system pressure thereby starving the pad of fluid and reducing or eliminating hydrodynamic lift-off or separation.
The present invention addresses some of these and/or other challenges associated with radial seal assemblies, including segmented radial seal assemblies.